1. Field of the Invention
The present invention relates to a leather-like sheet substrate having an excellent stretchability, and more particularly to a leather-like sheet substrate having a stretchability causing substantially no structural deformation even after repeated elongation, a good softness, a good drapeability and a dense feel.
2. Description of the Prior Art
Hitherto, artificial leathers have been used in various applications such as clothes, interiors, shoes, bags and globes. In particular, in the wearing applications such as clothes, shoes and globes, the sense of wearing and fitting comfort is required. Therefore, artificial leather materials for use in these applications have been strongly required to have a good stretchability and drapeability. However, in conventional artificial leathers having a sponge-like structure composed of a nonwoven fabric made of microfine fiber and a wet-impregnated resin, the dense feel and stretchability which are characteristic of leathers are contradictory to the drapeability. For example, if the dense feel is enhanced, the drapeability tends to be deteriorated. Therefore, there is a strong demand for developing an artificial leather simultaneously satisfying all of appearance, stretchability, dense feel and drapeability.
More specifically, the artificial leather is basically composed of an microfine fiber-entangled nonwoven fabric made of a non-elastic polymer such as polyamides and polyesters and a polymeric elastomer, typically a polyurethane, which is impregnated in the nonwoven fabric. Therefore, the fiber-entangled nonwoven fabric is subjected to only a limited range of structural deformation by elongation. If deformed by elongation beyond the limited range, the fiber-entangled nonwoven fabric may fail to restore its original shape. Although the polyurethane contained in the nonwoven fabric is stretchable, a maximum deformation of the artificial leather structure by elongation depends on the maximum deformation of the fiber-entangled nonwoven fabric. If the amount of the polymeric elastomer is increased, the resultant artificial leather loses its drapeability because of the repulsion of the polyurethane.
In view of these circumstances, various studies have been made to attain an excellent stretchability by forming a nonwoven fabric from fibers of an elastic polymer such as polyurethane. For example, there has been proposed a synthetic leather using a nonwoven fabric made of melt-blown polyurethane filaments (e.g., Japanese Patent No. 3,255,615, page 2). The proposed synthetic leather exhibits a good stretchability. However, the polyurethane filaments are limited in reducing their fineness and are inherently easy to stick together because of the tackiness of polyurethane. Therefore, the proposed synthetic leather is unusable in the applications such as suede in which the quality of appearance is largely affected by the fineness of fibers. Various studies have been made to reduce the tackiness of polyurethane itself in the technical fields other than the artificial leather art. For example, there have been proposed a method of preventing the sticking between polyurethane fibers by a lubricant (e.g., Japanese Patent No. 3,230,703, pages 2-3; Japanese Patent No. 3,230,704, page 2; and Japanese Patent Application Laid-Open No. 48-19893, pages 6-9), a method of preventing the sticking between polyurethane fibers by colloidal silica (e.g., Japanese Patent Application Laid-Open No. 60-239519, page 2), and a method of directly reducing the tackiness by blending another component to polyurethane (e.g., Japanese Patent Publication No. 47-36811, pages 1-2). The prevention of the sticking by a lubricant is effective for polyurethane fibers having a large fineness. However, the preventing effect is insufficient for microfine fibers having a fineness of 0.5 dtex or less which is required for producing an artificial leather having both good appearance and feel, thereby causing the sticking and thickening of microfine fibers. The stuck and thickened fibers are no longer restored to microfine fibers by the buffing for raising fibers. In the method of physically providing interstices between fibers by colloidal silica, when simply applied to microfine fibers, the sticking between the microfine fibers may occur with colloidal silica holding therebetween. If the particle size of colloidal silica is increased, the falling-off of colloidal silica held between microfine fibers becomes significant to result in the sticking of microfine fibers, thereby lessening the effect. The method of blending another component to polyurethane cannot simultaneously satisfy all the appearance, stretchability, dense feel and drapeability because the inherent stretchability of polyurethane is inhibited.